Is my gerbil pregnant or fat?

I got 2 gerbils a few weeks ago from PetCo and when I picked up my black one he/she? felt chubbier than normal. I'm pretty sure he's a guy but you never know...petco could just be stupid like that. Anyway my other gerbil a boy also I think he is pretty normal except he likes to chase the fat one around a lot. And they both cuddle a lot. I tried looking to see weather they were boys, but they are the really furry kind so it's hard to tell. Also when he lays down his sides look really big. Could he actually be a she or is he just fat?